Wireless communication method and system for communicating via multiple information streams

ABSTRACT

A wireless communication system is disclosed wherein an information source communicates with a mobile station via multiple intermediary base stations located in respective cells of a broadcast zone. In one embodiment, each base station sends multiple information streams that may be received by a mobile station located in the broadcast zone. By receiving multiple information streams from multiple base stations, the mobile station may enhance reception. In one embodiment, each base station in a broadcast zone transmits an information stream that exhibits a first level of robustness and another information stream that exhibits a second level of robustness.

RELATED PATENT APPLICATIONS

This patent application claims priority to Provisional U.S. PatentApplication by Rajkotia, et al., Ser. No. 60/748,725, filed Dec. 9,2005, entitled “Bandwidth Allocation Mechanism In The Multihop CellularNetworks”, which is assigned to the same assignee as the subject patentapplication and which is incorporated herein by reference in itsentirety.

This patent application is related to the U.S. Patent Application byKhan, et al., Ser. No. 11/554,726, entitled “Wireless CommunicationSystem And Methodology For Communicating Via Multiple InformationStreams”, filed concurrently herewith, which is assigned to the sameassignee as the subject patent application and which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The disclosures herein relate generally to wireless communicationsystems, and more particularly, to wireless communications systems thatemploy signal processing techniques to increase system capacity and moreeffectively use available bandwidth.

BACKGROUND

A significant challenge that communications equipment designers face ishow to pack more information into a given amount of radio frequencyspectrum or bandwidth. Frequency division multiplexing (FDM) is a knowntechnique for packing the transmission of data or information intoseveral closely-spaced channels or subcarriers within a predeterminedsignal bandwidth. FDM systems may separate subcarrier frequency spectraby using frequency guard bands to avoid interference among thesubcarriers. Unfortunately, this interference avoidance technique mayincrease system overhead and degrade bandwidth efficiency.

Orthogonal frequency division multiplexing (OFDM) provides a more robusttechnique for efficiently transmitting data using several subcarrierswithin a prescribed channel bandwidth. The OFDM method arranges thesubcarriers for greater efficiency as compared with the FDM method thatemploys guard bands. More particularly, OFDM overlaps the spectra of theOFDM subcarriers to more efficiently pack the subcarriers into theavailable channel bandwidth. However, to avoid interference among theOFDM subcarriers, the OFDM technique typically requires that thesubcarriers remain orthogonal to one another.

Many contemporary cellular communication systems employ OFDM technologyas a way to embed information on a radio frequency signal. Cellularsystems typically divide up a desired radio coverage area into a numberof smaller geographic areas referred to as cells. Each cell includes abase station generally located at or near the center of the cell. Thesystem assigns different radio frequencies to base stations in adjacentcells to avoid interference between adjacent cells. Mobile station userscommunicate with other mobile station users in the same or other cellsvia radio OFDM links through the base stations.

Cellular systems that employ OFDM may broadcast the same informationsimultaneously from all the cells of the system or from a subset of thecells. The cells or subset of cells form a broadcast zone. A mobilestation receiver in the broadcast zone may potentially receive signalfrom all cells in the broadcast zone. A single frequency network (SFN)may be formed by synchronizing all the cells in the broadcast zone andemploying OFDM as the communication mode. In such an SFN system, thesignal to interference plus noise ratio (SINR) may be improved because amobile station's receiver may collect the signal from all the cells inthe broadcast zone without interference except for background noise andsignals from other broadcast zones. This SFN OFDM technique may thusachieve improved recovery of broadcast information in comparison toother systems.

In conventional SFN OFDM systems, each base station in the broadcastzone may transmit a single stream of broadcast traffic. Unfortunately,the increase in broadcast traffic capacity with thesignal-to-interference-plus-noise ratio (SINR) is logarithmic. Thus, forlarger SINR, doubling of SINR results in a relatively low increase inbroadcast traffic capacity. Although reception is improved, this methodresults in an inefficient use of valuable radio frequency spectrum.

What is needed is a wireless communication system that addresses thebandwidth efficiency problems discussed above.

SUMMARY

Accordingly, in one embodiment, a wireless communication system isdisclosed that includes an information source that provides aninformation content stream. The system also includes a plurality of basestations that are coupled to the information source. Each base stationtransmits a respective first transmitted information stream derived fromthe information content stream and exhibiting a first level ofrobustness. Each base station also transmits a respective secondtransmitted information stream derived from the information contentstream and exhibiting a second level of robustness. The system furtherincludes a mobile receiving station that receives some of the firsttransmitted information streams to provide first received informationstreams and that further receives some of the second transmittedinformation streams to provide second received information streams. Themobile receiving station reconstructs the information content streamfrom both the first received information streams and the second receivedinformation streams if the mobile receiving station reliably receivesthe second transmitted information streams. Otherwise, the mobilestation reconstructs the information content stream from the firstreceived information streams.

In another embodiment, a method is disclosed for wirelessly transmittingand receiving information. The method includes providing, by aninformation source, an information content stream to a plurality of basestations situated in respective cells. The method also includestransmitting, by the plurality of base stations, respective firsttransmitted information streams derived from the information contentstream and exhibiting a first level of robustness. The method furtherincludes transmitting, by the plurality of base stations, respectivesecond transmitted information streams derived from the informationcontent stream and exhibiting a second level of robustness. The methodstill further includes receiving, by a mobile receiving station, some ofthe first transmitted information streams to provide first receivedinformation streams. The method also includes receiving, by the mobilereceiving station, some of the second transmitted information streams toprovide second received information streams. The method further includestesting, by the mobile receiving station, the second receivedinformation streams to determine if the mobile receiving stationreliably received the second received information streams. The methodalso includes reconstructing, by the mobile receiving station, theinformation content stream from the first received information streamsand the second received information streams if the testing stepdetermined that the mobile receiving station reliably received thetransmitted second information streams. Otherwise, the mobile receivingstation reconstructs the information content stream from the firstreceived information streams.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended drawings illustrate only exemplary embodiments of theinvention and therefore do not limit its scope, because the inventiveconcepts lend themselves to other equally effective embodiments.

FIG. 1 shows a block diagram of a transmitter and receiver of a wirelesscommunication device.

FIG. 2 is a graphic representation of on OFDM signal.

FIG. 3 shows multiple cells in a broadcast zone of a wirelesscommunication system.

FIG. 4A is a block diagram of one embodiment of the disclosed wirelesscommunication system including multiple base stations and a mobilestation.

FIG. 4B is a block diagram of a representative mobile receiving stationof the system of FIG. 4A.

FIG. 5 is a block diagram of another embodiment of the disclosedwireless communication system including multiple base stations and amobile station.

FIG. 6 shows a block diagram of an alternative base station usable inone embodiment of the disclosed wireless communication system.

FIG. 7 shows a block diagram of another alternative base station usablein one embodiment of the disclosed wireless communication system.

FIGS. 8A and 8B depict a broadcast/multicast pilot symbol mapping to anorthogonal time frequency resource for transmission from two basestation antennas in the disclosed wireless communication system.

FIG. 9 shows a block diagram of yet another alternative base stationusable in one embodiment of the disclosed wireless communication system.

FIG. 10 shows a block diagram of still another alternative base stationusable in one embodiment of the disclosed wireless communication system.

FIG. 11 shows a block diagram of an alternative base station usable inone embodiment of the disclosed wireless communication system.

FIG. 12 shows a block diagram of an alternative base station usable inone embodiment of the disclosed wireless communication system.

FIG. 13 is a flowchart that shows stream interference cancellationmethodology that a receiver in a mobile device may employ in thedisclosed wireless communication system.

FIG. 14 shows a block diagram of another alternative base station usablein one embodiment of the disclosed wireless communication system.

FIG. 15 is a flowchart that depicts reception and decoding of a baselayer and an enhanced layer transmitted by base stations in thedisclosed wireless communication system.

FIG. 16 shows a block diagram of yet another alternative base stationusable in one embodiment of the disclosed wireless communication system.

FIG. 17 shows a block diagram of still another alternative base stationusable in one embodiment of the disclosed wireless communication system.

FIG. 18 shows a block diagram of another base station usable in oneembodiment of the disclosed wireless communication system.

FIG. 19 shows a block diagram of another embodiment of the disclosedwireless communication system.

DETAILED DESCRIPTION

FIG. 1 shows a base station 100 that is usable in a conventionalOFDM-based wireless communication system. Base station 100 includes bothan OFDM transmitter 105 and an OFDM receiver 110. A mobile station maybe similarly configured to communicate with base station 100. A datasource 115 provides information or data to a quadrature amplitudemodulator (QAM) circuit 120 that generates QAM modulated symbols at theQAM circuit output. A serial to parallel converter 125 converts the QAMmodulated symbols to a parallel form, namely a series of sub-symbols. Aninverse fast Fourier transform (IFFT) stage 130 receives the parallelconverted signals and transforms these signals from the frequency domainto the time domain. IFFT stage 130 generates N time domain samples atits output, wherein N refers to the IFFT/FFT size used by the OFDMcommunication system. A parallel to serial converter 135 couples to theoutput of IFFT stage 130 to convert the time domain signals it receivestherefrom to a serial format.

An add cyclic prefix (CP) stage 140 couples to the output of parallel toserial converter 135 to add a cyclic prefix to the signal sequence itreceives from the parallel to serial converter 135. The resultingsequence of signals at the add CP prefix stage output is referred to asan OFDM symbol 200, such as shown in FIG. 2. OFDM symbol 200 includesdata 205 and a cyclic prefix 210.

Returning to FIG. 1, receiver 110 includes a cyclic prefix (CP) removalstage 150 that removes the cyclic prefix from the OFDM signal that itreceives. A serial to parallel converter stage 155 converts the signalfrom CP removal stage 150 to a parallel format. A fast Fourier transform(FFT) stage 160 receives the parallel converted signal in the timedomain and transforms that signal to the frequency domain at the outputof FFT stage 160. A parallel to serial converter 165 converts the outputsignal of FFT stage 160 from a parallel format to a serial format. Theresulting QAM modulated symbols feed from converter 165 to a QAMdemodulator 170. QAM demodulator 170 demodulates the QAM symbols intodata 175.

FIG. 3 shows cells 1-19 that together form a broadcast zone 300 of aconventional wireless communications system. Each cell includes a basestation, tower and antenna (not shown) typically located at the centerof the cell. Cells 1-19 are a subset of all of the cells of thecommunication system. In one possible system configuration, all cells1-19 of the subset of cells in the broadcast zone simultaneouslytransmit the same information content. Thus a receiver, such as mobilestation 305, listening to the broadcast content may potentially receivesignals from all of the cells in the broadcast zone. If the systememploys OFDM for transmission, and if all of the cells in the broadcastzone are synchronized, a single frequency network (SFN) may be formed.In such an SFN system, a receiver in mobile station 305 may collectsignal from all of the cells of the broadcast zone without interferenceexcept for background noise and any interference from the cells notbelonging to the broadcast zone. This topology facilitates betterrecovery of the broadcast information due to improvement in thesignal-to-interference-plus-noise ratio (SINR).

In the conventional communications system discussed above with referenceto FIG. 3, each base station in the respective cells 1-19 of thebroadcast zone transmits a single stream of broadcast traffic. Thisresults in a logarithmic increase in broadcast traffic capacity becausefor the very high SINR of this SFN system, the increase in capacity withSINR is logarithmic as given by Shannon's capacity formula, Equation 1:Capacity=log₂(1+SINR)b/s/Hz  Equation 1However, for SINR much greater than 1, the capacity is given by Equation2:Capacity=log₂(SINR)b/s/Hz (for SINR>>1)  Equation 2Thus, for larger SINR, doubling of SINR results in only a 1 b/s/Hzincrease in capacity for this SFN system. For this reason, aconventional SFN system that employs a single broadcast stream resultsin inefficient use of scarce radio frequency spectrum.

FIG. 4A shows one embodiment of the disclosed wireless communicationsystem 400 wherein multiple base stations or cells each transmitmultiple broadcast streams over multiple antennas. More particularly,system 400 includes master stations 401, 402 and 403, each masterstation being situated in a respective cell (not shown). Other systemsare contemplated wherein the system employs two master stations, or morethan three master stations, depending on the particular application. Inthis embodiment, system 400 exhibits a multiple-input multiple-output(MIMO) configuration in that multiple streams are transmitted bymultiple antennas.

System 400 includes a broadcast/multicast content server 410 thatprovides information or content to be broadcast or multicast by thesystem. Server 410 couples to a broadcast/multicast controller 415 toprovide controller 415 with the information. The coupling between server410 and controller 415 may be either wired or wireless depending on theparticular application. Controller 415 couples to base stations 401, 402and 403 to provide the information, now designated as information 420,to these base stations in their respective cells. The coupling betweencontroller 415 and base stations 401, 402 and 403 may be either wired orwireless depending on the particular application. For purposes of thisdocument, the term “wired” includes electrical conductors, opticalconductors and other physical conductors. In one embodiment,broadcast/multicast content server 410 and broadcast/multicastcontroller 415 are coupled to, and controlled by, a central controlfacility (not shown) located either inside or outside of the broadcastzone formed by cells 401-403. The coupling between the central controlfacility and server 410/controller 415 may itself be either wired onwireless.

Base stations 401, 402 and 403 each receive the same information 420 andperform the same signal processing operations on that information. Byway of example, base station 401 includes a coding and modulation stage425 that channel codes and modulates the information. In one embodiment,the coding performed by coding and modulation stage 425 adds redundancyto the information transmitted by the base station to improve thereliability of transmission. The modulation function performed by codingand modulation stage 425 determines how the coded information bitsmodulate the RF carrier transmitted by the base station. Moreparticularly, in one embodiment, coding and modulation stage 425 employsOFDM to modulate the information on a radio frequency signal. Ademultiplexer (DMUX) 430 couples to coding and a modulation stage 425.DMUX 430 demultiplexes the signal it receives into multiple parallelstreams. The number of parallel streams that base station 401 transmitsdepends on the number of available antennas in the base station. In thisparticular example, DMUX 430 demultiplexes the signal into two parallelstreams, namely stream 431 and stream 432. After further processing bystages 431A-431D, streams 431 and 432 are provided to two antennasrespectively, namely antenna 441 and antenna 442. In another embodiment,if base station 401 employs three antennas, then demultiplexer 430 wouldprovide three streams to those three antennas, respectively. In asimilar manner, DMUX 430 may demultiplex the signal into more than threestreams provided that base station 401 includes a respective antenna foreach stream.

More detail is now provided with respect to the processing that stages431A-431D perform on stream 431. One output of demultiplexer DMUX 431couples to CP stage 431A such that CP stage 431 adds a cyclic prefix tostream 431. Digital to analog (D/A) conversion and filtering areprovided to stream 431 by D/A converter 431B and filter 431C,respectively. A radio frequency (RF) amplifier stage 431D couples tofilter 431C to provide RF amplification to the filtered RF signal thatfilter 431C provides to RF amplifier 431D. The resultant amplified RFsignal feeds antenna 441 for transmission thereby. In a similar manner,stages 432A-432D process stream 432 before providing the resultantamplified RF signal to antenna 442.

Base stations 402 and 403 exhibit substantially the same circuittopology as base station 401, as seen in FIG. 4A. More particularly,coding and modulation stage 445, DMUX 450, stream 451, stream 452,antenna 461 and antenna 462 of base station 402 correspond respectivelyto coding and modulation stage 425, DMUX 430, stream 431, stream 432,antenna 441 and antenna 442. For simplicity in the FIG. 4A drawing, theCP stages, D/A stages, filter stages and RF amplifiers are not shown inbase station 402 or base station 403, although these components may beemployed in actual practice as shown in base station 401. In a similarmanner, the components of base station 403 correspond to respectivecomponents of base station 401 and base station 402.

As described above, all base stations 401, 402 and 403 receive the sameinformation to transmit from broadcast/multicast controller 415. Eachbase station splits the coded and modulated information into twoparallel first and second information streams. Antennas 441, 461 and 481transmit substantially identical first information streams 431, 451 and471, respectively. Antennas 442, 462 and 482 transmit substantiallyidentical second information streams 432, 452 and 472, respectively. Inthis manner, information streams 431, 451 and 471 form correspondingstreams. Information streams 432, 452 and 472 also form correspondingstreams.

Because substantially the same information stream is transmitted bycorresponding antennas 441, 461 and 481 of all the base stations, thetransmissions from the multiple base stations appear as multipathtransmissions to a mobile station 490 receiving the broadcast/multicastcontent. The mobile station or receiver 490 includes two receiveantennas, namely antenna 491 and antenna 492. If all base stationstransmitting the broadcast content include at least two transmitantennas, then two spatially-multiplexed streams can be transmitted tomobile receivers having at least two antennas, such as mobile station490. If the base stations include more antennas and respective streamsthan the two shown, receiver 490 may includes a larger number ofantennas than two to accommodate all transmitted information streams,namely one antenna per information stream. It is noted that antennas442, 462 and 482 also transmit substantially the same information.Within each antenna pair per base station, for example antenna pair 441,442, each antenna exhibits spatial diversity with respect to the other.Likewise antennas 491, 492 exhibit spatial diversity in the receiver ofmobile station 490.

FIG. 4B shows a block diagram of a representative receiver that may beused as a receiver for mobile station 490. Mobile station receiver 490includes a demodulation stage 494 that couples to antennas 491 and 492.Demodulation stage 494 demodulates whatever modulation type is employedin base stations 401, 402 and 403. In the example of FIG. 4A whereincoding and modulation stage 425 employs QAM modulation, thendemodulation stage 494 demodulates QAM signals. In further embodimentsdescribed below wherein the base stations employs QPSK modulation, thendemodulation stage 494 demodulates QPSK signals. In yet otherembodiments described below, wherein the base stations transmit signalstreams of different modulation types, for example one stream of QAMsignals and another stream of QPSK signals, then demodulation stage 494demodulates signals streams of each modulation type. In other words,demodulation stage 494 demodulates a received QAM modulated informationstream and a received QPSK information stream. Mobile station receiver490 also includes a decoder 496 coupled to the demodulation stage 494.Decoder 496 decodes whatever coding type was employed by coding andmodulation stage 425 in the base stations to code the informationstreams to be transmitted. Representative coding types that may beemployed by the base stations to code information streams include turbocoding, low density parity check (LDPC) coding and unitary pre-coding,as discussed in more detail below. Depending on the type of codingemployed by the base stations, the decoder 496 of the mobile receivingstation 490 is selected to decode that type of coding. The decodedinformation content is provided to receiver output 498.

FIG. 5 shows another embodiment of the disclosed wireless communicationsystem as system 500. System 500 includes elements in common with system400, discussed above. In comparing system 500 of FIG. 5 with system 400of FIG. 4A, like numbers indicate like elements. In system 500, each ofthe information streams transmitted by the multiple antennas shown isseparately encoded and modulated. In more detail, system 500 includesbase stations 501, 502 and 503. By way of example, base station 501includes a demultiplexer, DMUX 510, which separates information providedthereto into two information streams 511 and 512. DMUX 510 includes twooutputs that couple to coding and modulation stages 520 and 525,respectively, as shown. In this manner, each information stream isprovided with a respective dedicated coding and modulation stage. Codingand modulation stage 520 channel codes and modulates information stream511, thus generating a coded modulated information stream 511′ at itsoutput. Similarly, coding and modulation stage 525 channel codes andmodulates information stream 512, thus generating a coded modulatedinformation stream 512′ at its output. Base station 501 includesantennas 531 and 532 that transmit coded modulated information streams511′ and 512′, respectively. In a manner similar to that of base station401 of FIG. 4A, each of the information streams of base station 501 isprocessed by respective CP stages, D/A stages, filter stages and RFamplifiers (not shown) before being applied to respective antennas 531and 532.

Base stations 502 and 503 exhibit substantially the same topology asbase station 501, as shown in FIG. 5. More particularly, base station502 includes a DMUX 540, a coding and a modulation stage 550, a codingand a modulation stage 555, and antennas 561, 562, that correspondrespectively to the base station 501 components DMUX 510, coding andmodulation stage 520, coding and modulation stage 525, and antennas 531,532. In a similar manner, the components of base station 503 correspondto respective components of base station 501 and base station 502, asshown. The topology of system 500 provides for separate coding andmodulation of multiple information streams in each base station, namelytwo information streams in this particular example. In one embodiment,the information content is targeted at multiple mobile stations in amulti-cast/broadcast transmission. For this reason, it is typically notpossible to adapt the modulation and coding for the informationtransmitted over multiple antennas. A fixed modulation and coding schememay be used for transmission from multiple antennas, as shown. However,each of the transmitted information streams may be separately cyclicredundancy check (CRC) protected and encoded/modulated, as shown in FIG.6 discussed below.

FIG. 6 shows a representative base station 601 that applies CRCprotection to each of multiple information streams and which separatelyencodes and modulates each of these multiple information streams. Basestation 601 may be employed as an alternative to base stations 501, 502and 503 of FIG. 5. In more detail, a demultiplexer (DMUX) 605 inrepresentative base station 601 receives a broadcast information stream607. DMUX 605 divides or splits that information stream into multipleinformation streams, namely information streams 611 and 612 in thisparticular example. A CRC attachment stage 615 applies a CRC code tostream 611. A turbo coding/low density parity check (LDPC) coding stage620 couples to CRC attachment stage 615. Stage 620 either turbo codes orLDPC codes the information stream provided thereto. A turbo code is aclass of concatenated error-control coding methods that offer highperformance while requiring just moderate complexity. An iterativeprinciple may be employed for decoding turbo codes. In a manner similarto turbo codes, LDPC codes also employ an iterative decodingmethodology. LDPC codes are constructed using sparse random parity checkmatrices. A modulation stage 625 couples to turbo/LDPC coding stage 620to modulate a radio frequency signal with the coded information streamthat it receives from turbo/LDPC coding stage 620. Modulation stage 625may employ quadrature phase shift keying (QPSK) or quadrature amplitudemodulation (QAM) to perform such modulation. Modulation stage 625generates a modulated signal that is provided to antenna 631 fortransmission.

CRC attachment stage 615, turbo/LDPC coding stage 620, modulation stage625 and antenna 631 together form a signal path for processinginformation stream 611. In a similar manner, CRC attachment stage 645,turbo/LDPC coding stage 650, modulation stage 655 and antenna 632 form asignal path for processing information stream 612. Thus, antennas 631and 632 both transmit respective information streams of common contentmodulated on OFDM signals. A wireless communication system using thebase station technology of FIG. 6 may exhibit a system topology likethat of system 400 of FIG. 4A, except that base stations 601 aresubstituted for each of base stations 401, 402 and 403. In actualpractice, base station 600 may employ additional signal processingstages as shown in FIG. 4A, such as a CP stage 431A, D/A stage 431B,filter stage 431C and RF stage 431D, between modulation stage 625 andantenna 631. Similar additional processing stages may be employedbetween modulation stage 655 and antenna 632.

FIG. 7 shows a base station 701 that may be employed as an alternativeto base stations 501, 502 and 503 of FIG. 5. In this embodiment, DMUX705 demultiplexes information into an information stream 711 and aninformation stream 712. CRC/coding/modulation stage 721 attaches a CRCcode, encodes, and modulates information stream 711 to generate codedstream 711. The coding in stage 721 is performed using a channel codersuch as turbo code or LDPC code. The modulation in stage 721 may beperformed by using QAM, QPSK or any other suitable modulation scheme. Aninverse fast Fourier transform (IFFT) circuit 731, such as a digitalsignal processor (DSP), performs an inverse fast Fourier transform oninformation stream 711′ and a PILOT1 signal to convert the codedinformation stream 711′ and PILOT1 from the frequency domain to the timedomain, thus generating a converted coded information stream 711″ at itsoutput. An add cyclic prefix (CP) circuit 741 couples to the output ofIFFT circuit 731 to attach a cyclic prefix to converted codedinformation stream 711″, thus generating an information stream 711′″that is transmitted by antenna 751. In actual practice, a D/A converter,filter and RF amplifier (not shown) may be situated between add CPcircuit 741 and antenna 751 to further process information stream 711′in a manner similar to stages 431B-431D of FIG. 4A.

CRC/coding/modulation stage 721, IFFT circuit 731, add CP circuit 741and antenna 751 together form a signal path for processing theinformation stream 711 provided by DMUX 705. In a similar manner,CRC/coding/modulation stage 722, IFFT circuit 732, add CP circuit 742and antenna 752 together form a signal path for processing informationstream 712 provided by DMUX 705. In actual practice, a D/A converter,filter and RF amplifier (not shown) may be situated between add CPcircuit 742 and antenna 752 to further process information stream 712′″in a manner similar to stages 431B-431D of FIG. 4A. In the latter signalpath for information stream 712, the pilot signal, PILOT 2, isorthogonal to pilot signal PILOT 1. Thus, in this embodiment, orthogonalpilot signals are transmitted for each of antennas 751 and 752 of agiven base station. The pilot or reference signals PILOT 1 and PILOT 2are sequences known at the receiver 490. Receiver 490 compares thereceived pilot signals with a stored known pilot sequence to determinechannel estimates. The channel estimates for PILOT 1 and PILOT 2 areused by receiver 490 to demodulate and decode information stream 711 andinformation stream 712 respectively. As stated above, a broadcast zonemay have multiple base stations, each with multiple antennas as shown inFIG. 7. The pilot signals for multiple antennas in multiple basestations in a broadcast zone may use the same time-frequency resourceshown in FIGS. 8A and 8B.

FIGS. 8A and 8B depict a broadcast/multicast pilot symbol mapping to anorthogonal time-frequency resource for transmission from two basestation antennas, namely antenna 751 in FIG. 8A and antenna 752 in FIG.8B. For simplicity, antenna 751 is designated as antenna 1 and antenna752 is designated as antenna 2. The horizontal axis shows time that isdivided into subframes of 0.5 ms each. The vertical axis showsfrequency. In the symbol mapping of FIG. 8A, B1 designates the pilotsignal, PILOT 1, and in FIG. 8B, B2 designates the pilot signal PILOT 2.In the time-frequency resource mapping wherein multiple base stations ina broadcast zone transmit PILOT 1 (B1), no signal is transmitted onantennas 2 of the base stations. Conversely, antennas 1 of the multiplebase stations in the broadcast zone transmit no signal on time-frequencylocations wherein PILOT 2 (B2) is transmitted, as seen by comparing FIG.8A and FIG. 8B. In one embodiment, a time-frequency resource consists ofa set of OFDM subcarrier frequencies over a given time interval such asa subframe of 0.5 ms. Two different scrambling codes may be employed forantennas 1 and 2. The scrambling codes are applied before the IFFTstages in FIG. 7 to stream 711′ and 712′ and also to PILOT 1 and PILOT2. A scrambling code generally is a pseudo-random number (PN) sequencethat is pre-known at the receiver and stored in a memory (not shown)therein.

FIG. 9 shows an embodiment of a base station 901 that transmitsbroadcast/multicast streams over multiple transmit antennas wherein thestreams are unitary pre-coded before stream mapping to the antennas. Inactual practice, multiple base stations 901 in respective cells areemployed to cover a particular broadcast zone. In this embodiment, eachof the information streams is potentially transmitted from all of theantennas used in the broadcast/multicast information transmission. Thisunitary pre-coding technique can be used in both cases wherein theinformation streams are separately encoded and modulated, and alsowherein the information streams are jointly encoded and modulated.

Base station 901 includes many elements in common with base station 601of FIG. 6, namely the multiplexer 605, attach CRC stages 615, 645, turbocoding/LDPC coding stages 620, 650, modulation stages 625 and 655, andantennas 631 and 632. STREAM 1 refers to information transmitted by theupper signal path formed by attach CRC stage 615, turbo/LDPC codingstage 620, modulation stage 625, unitary pre-coding stage 905 andantenna 631. STREAM 2 refers to the information stream transmitted bythe lower signal path formed by attach CRC stage 645, turbo/LDPC codingstage 650, modulation stage 655, unitary pre-coding stage 905 andantenna 632.

Unitary pre-coding stage 905 performs unitary pre-coding on the twoinformation streams provided thereto before these information streamsare transmitted by antennas 631 and 632. Two examples of unitarypre-coding matrices, P1 and P2, for the two antennas per base stationare:

${{P\; 1} = {\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}}},{{P\; 2} = {\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}}}$Unitary pre-coding stage 905 receives modulated symbols S1 and S2 frommodulation stages 625 and 655, respectively, as shown in FIG. 9. Unitarypre-coding stage 905 pre-codes the modulated symbols S1 and S2 withpre-coding matrices P1 and P2. Assuming modulation symbols S1 and S2 aretransmitted at any given time from stream 1 and stream 2, respectively,then modulation symbols after pre-coding with matrix P1 and P2 may bewritten as:

${T\; 1} = {{P\;{1\left\lbrack \frac{S\; 1}{S\; 2} \right\rbrack}} = {{{\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}} \times \begin{bmatrix}{S\; 1} \\{S\; 2}\end{bmatrix}} = {\frac{1}{\sqrt{2}}\begin{bmatrix}{{S\; 1} + {S\; 2}} \\{{S\; 1} - {S\; 2}}\end{bmatrix}}}}$${T\; 2} = {{P\;{2\left\lbrack \frac{S\; 1}{S\; 2} \right\rbrack}} = {{{\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}} \times \begin{bmatrix}{S\; 1} \\{S\; 2}\end{bmatrix}} = {\frac{1}{\sqrt{2}}\begin{bmatrix}{{S\; 1} + {j\; S\; 2}} \\{{S\; 1} - {j\; S\; 2}}\end{bmatrix}}}}$Antennas 631 and 632 of base station 901 will respectively transmit thefollowing pre-coded signals, T11 and T12, when unitary pre-coding stage905 uses P1 as the pre-coding matrix:

${T\; 11} = \frac{\left( {{S\; 1} + {S\; 2}} \right)}{\sqrt{2}}$${T\; 12} = \frac{\left( {{S\; 1} - {S\; 2}} \right)}{\sqrt{2}}$However, antennas 631 and 632 of base station 901 will respectivelytransmit the following pre-coded signals, T₂₁ and T₂₂, when unitarypre-coding stage uses P2 as the pre-coding matrix:

${T\; 21} = \frac{\left( {{S\; 1} + {j\; S\; 2}} \right)}{\sqrt{2}}$${T\; 22} = \frac{\left( {{S\; 1} - {j\; S\; 2}} \right)}{\sqrt{2}}$

FIG. 10 shows another base station 1001 usable as a base station in abroadcast zone. Base station 1001 employs two modulation types, onemodulation type exhibiting a level of robustness greater than the othermodulation type. For example, base station 1001 employs QPSK modulationas the more robust modulation type and QAM 16 as the less robustmodulation type. By robustness is meant that some communicationsmechanisms or methods exhibit a higher level of signal quality thatother mechanisms or methods. For example, QPSK typically exhibitsgreater signal quality or level of robustness that QAM. Some mechanismsor methods are less susceptible to interferers than other mechanisms andthus exhibit a higher level of robustness. The topology of base station1001 is similar to that of base station 601 of FIG. 6 with like numbersindicating like components. Attach CRC stage 615, turbo/LDPC codingstage 620, QPSK modulation stage 1005 and antenna 631 form a signal paththat processes information stream STREAM 1. Attach CRC stage 645,turbo/LDPC coding stage 650, 16QAM modulation stage 1015 and antenna 632form a signal path that processes information stream STREAM 2.

As described above, base station 1001 employs a modulation type inmodulator 1005 for the information stream STREAM 1 that is more robustthan the modulation type employed by modulation stage 1015 for theinformation stream STREAM 2. In this representative embodiment,modulation stage 1005 is a QPSK modulator and modulation stage 1015 is a16-QAM modulator. QPSK modulator 1005 transmits information in a morerobust signalling format than 16-QAM modulator 1015. DMUX 605 dividesthe broadcast information into two information streams, namely STREAM 1and STREAM 2, as shown. One output of DMUX 605 is series to parallelconverted to generate information stream STREAM 1 that exhibits a datarate, R, bit/sec. The remaining output of DMUX 605 is series to parallelconverted to generate information stream STREAM 2 that exhibits a datarate of 2×R bits/sec. However, in this particular embodiment, themodulation symbol rates transmitted by antennas 631 and 632 are thesame, namely K symbols/sec, as indicated in FIG. 10. Modulation stage1005 employs QPSK modulation with 2 bits/symbol for information streamSTREAM 1 and modulation stage 1015 employs 16-QAM modulation forinformation STREAM 2, thus resulting in the same modulation symbol rate,K symbols/sec, being transmitted by both antennas 631 and 632.

In yet another embodiment, FIG. 11 shows a base station 1101 that iscapable of transmitting information over multiple transmit antennaswherein coding stage 1120 employs a more robust coding for theinformation stream STREAM 1 than the coding that coding stage 1150employs for the information stream STREAM 2. In this particular example,coding stage 1120 employs a coding rate of ⅓ and coding stage 1150employs a coding rate of ⅔. A coding rate is a ratio between the numberof input bits to an encoder and the number of output bits from theencoder. A coding rate of ⅓ means that the number of bits at the outputof the encoder is 3 times larger than the number of input bits to theencoder. The coding rate of ⅓ employed by coding stage 1120 is morerobust or reliable than the coding rate of ⅔ employed by coding stage1150. Coding rates other than these coding rates, given for purposes ofexample, may also be employed as long as the coding rate of one codingstage is selected to be more robust than the coding rate of the othercoding stage.

In a manner similar to base station 1001 of FIG. 10, the serial toparallel converted signals at the two outputs of DMUX 605 exhibit Rbits/sec for information stream STREAM 1 and 2×R bits/sec forinformation stream STREAM 2. However, the modulation symbol rateactually transmitted by antennas 631 and 632 is the same, namely Ksymbols/sec, because coding stage 1120 codes information stream STREAM 1using a ⅓ coding rate (R=⅓) while coding stage 1150 codes informationstream STREAM 2 using a ⅔ coding rate (R=⅔).

FIG. 12 shows yet another embodiment of a base station 1201 usable asthe base stations of a broadcast zone wherein one antenna of the basestation transmits at a higher power level than the other antenna of thebase station. Base station 1201 includes a unitary pre-code stage 1205that pre-codes modulated symbols S1 and S2 in information streamsSTREAM1 and STREAM 2 in a manner similar to unitary pre-coding stage 905of FIG. 9. However, base station 1201 includes radio frequency (RF)power amplifiers (PAs) 1215 and 1220 that respectively amplify themodulated S1 signals in STREAM 1 and the modulated S2 signals in STREAM2. In this embodiment, RF PA 1215 exhibits higher power gain than RF PA1220. Thus, antenna 631 transmits information stream STREAM 1 at ahigher power level than antenna 632 transmits information stream STREAM2. In practice, the broadcast zone may include many base stations suchas 1201, each within its respective cell. In such a system, each of themultiple antennas 631 that transmit STREAM 1 exhibit a higher radiatedpower output than each of the antennas 632 that transmit the STREAM 2.This multiple level transmit power configuration with higher power forSTREAM 1 than STREAM 2 makes decoding STREAM 1 by a mobile receiver 490receiving the broadcast content more reliable. A higher transmit poweron an information stream translates into a higher signal quality forthat stream when received by a receiver such a receiver 490.

FIG. 13 is a flowchart which shows stream interference cancellation thatreceiver 490 of FIGS. 4-7 and FIGS. 9-12 may employ. In this example,the base stations transmit information STREAM 1 employing a more robustmodulation type, a more robust coding or a higher power than informationstream STREAM 2, as described in the above representative embodiments ofFIGS. 10, 11 and 12, respectively. As seen in the flowchart of FIG. 13,mobile station receiver 490 first decodes the information stream STREAM1 that exhibits the more robust modulation type, more robust coding orhigher power than the remaining stream or streams, as per block 1305.The receiver 490 then determines a channel estimate that is derived fromreference pilot signals in the received signal, as per block 1310.Although not specifically shown, the transmitters depicted in FIGS. 4-7and FIGS. 9-12 may employ pilot signals to enable coherent demodulationby the receivers shown in FIGS. 4-7 and FIGS. 9-12. These pilot signals,which may also be called reference signals, are sequences that are knownat the receiver. After determining a channel estimate, the receiver 490reconstructs the signal for information stream STREAM 1 based on thechannel estimate derived from the received reference pilot signals, asper block 1315, to provide a reconstructed STREAM 1. Next, the receiver490 cancels the reconstructed STREAM 1 from the overall received signal,as per block 1320, thus leaving a received STREAM 2 remaining aftercancellation. Receiver 490 then uses the signal that results from thiscancellation to decode the information STREAM 2 received by receiver490, as per block 1325. The overall received signal may be also referredto as a composite signal because it may include both STREAM1 and STREAM2.

FIG. 14 is a block diagram that depicts a broadcast content server 1400coupled to a base station 1401. In actual practice, content server 1400may be coupled to several base stations like base station 1401, eachbase station being situated in its own cell. Broadcast content server1400 splits the content into information streams STREAM 1 and STREAM 2at the respective outputs of server 1400. Information stream STREAM 1includes a base layer of information and information stream STREAM 2includes an enhanced layer of information as shown. The base layer andthe enhanced layer are processed by respective signal paths in the basestation as described below, before ultimately being transmitted byantennas 1441 and 1442, respectively. Base station 1401 includes anattach CRC stage 1411, a turbo/LDPC coding stage 1421, a QAM modulationstage 1431 and an antenna 1441, that together form a signal path for thebase layer of information STREAM 1. Base station 1401 also includes anattach CRC stage 1412, a turbo/LDPC coding stage 1422, a QAM modulationstage 1432 and an antenna 1442, that together form a signal path for theenhanced layer of information STREAM 2.

The base layer and enhanced layer each carry the same broadcast programsuch as a video streaming application, for example. The base layer ofSTREAM 1 may include a relatively low quality audio feed and relativelylow resolution video, whereas the enhanced layer of STREAM 2 may includea relatively high quality audio feed and a relatively high resolutionvideo. In other words, the enhanced layer may include higher resolutionaudio, video and/or other information as compared with the base layer.The enhanced layer may carry additional information to enhance the audioand video quality of a video streaming application that provides orhandles content in server 1405.

All mobile station receivers 490 in a broadcast zone of base stationsdecode the base layer upon reception of a signal containing the baselayer in the broadcast content. However, in one embodiment, only thosemobile station receivers 490 currently experiencing more that apredetermined level of channel quality will decode the enhanced layer.To make this determination with respect to the quality of the receivedsignal, receiver 490 may include a channel quality indicator circuit495. A signal to interference plus noise ratio (SINR) circuit is oneexample of a device for determining the quality of a received signalthat receiver 490 may employ as channel quality indicator circuit 495.By decoding the enhanced layer, those mobile stations experiencing morethan the predetermined channel quality level, namely those mobilestations relatively close to the base station, may provide higherquality received video and audio or other information.

As seen in FIG. 14, the base layer of information STREAM 1 exhibits adata rate of R bits per second, while the enhanced layer of informationSTREAM 2 exhibits a data rate of 2×R bits per second. Thus, a mobilestation receiver 490 that receives only the base layer may receive adata rate of R bits per second. However, a mobile station receiver 490that successfully decodes the enhanced layer as well as the base layereffectively receives the broadcast content at a rate 3 times higher thanthe base layer alone or 3×R bits per second.

In a one embodiment, modulation stages 1431 and 1432 are 16-QAMmodulation stages. Thus, base station 1401 transmits both the base layerand the enhanced layer in 16-QAM modulation. However, in one embodiment,base station 1401 may employ a more robust coding rate of ⅓ for the baselayer and a relatively less robust coding rate of ⅔ for the enhancedlayer. More particularly, turbo/LDPC coding stage 1421 applies a morerobust coding rate of R=⅓ to the base layer and turbo/LDPC coding stage1422 applies a less robust coding rate of R=⅔ to the enhanced layer.Moreover, in one embodiment, reception of the base layer is made morereliable by using a more robust modulation type such as QPSK inmodulation stage 1431 while modulation stage 1432 uses less robust QAMmodulation for the enhanced layer. Reception of the base layer may alsobe made more reliable by using higher RF power to transmit the baselayer from antenna 1441 than the RF power at antenna 1442. An RFamplifier (not shown) may be situated between modulation stage 1431 andantenna 1441 for this purpose along with another RF amplifier (notshown) between modulation stage 1432 and antenna 1442. In that scenario,the RF amplifier coupled to antenna 1441 exhibits a higher RF outputpower or gain than the RF amplifier coupled to antenna 1442. In anotherembodiment, a unitary pre-coding stage such as stage 905 of FIG. 9 maybe used at the outputs of modulation stages 1431 and 1432 to pre-codethe base layer and the enhanced layer with a pre-coding matrix toincrease reception reliability. In yet another embodiment, base station1401 may scramble the base layer and the enhanced layer with differentscrambling codes to randomize inter-stream interference. The transmittermay perform this scrambling operation after modulation and beforemapping the symbols to an input of an IFFT (not shown) in thetransmitter. A scrambling code generally is a pseudo-random number (PN)sequence that is pre-known at the receiver. For example, the receiverstores the pseudo-random sequence in a non-volatile memory therein. Thetransmitter multiplies the transmitted symbols with the scrambling code,namely the pseudo-random sequence. The descrambling at the receiver isachieved by correlating the received modulation symbols with thescrambling sequence used by the transmitter. It is noted that each ofabove the techniques for increasing the reliability of reception may beused in combination with one another to increase the reliability ofreception by a mobile station receiver.

FIG. 15 is a flowchart that shows the methodology that mobile station490 of FIG. 14 employs to receive the base layer and enhanced layertransmitted by base station 1401. Process flow commences with startblock 1505. All mobile stations 490 in the broadcast zone first decodethe base layer, as per block 1510. As described above, base station 1401transmits the base layer with high reliability namely with more robustcoding, more robust modulation, and/or higher power in comparison withthe coding, modulation or power used to transmit the enhanced layer.Receiver 409 performs a channel estimate, as per block 1515. Thereceiver uses such channel estimates for reconstructing the decoded baselayer for cancellation from the overall received signal. All mobilestations 409 receiving a broadcast stream in the broadcast zone firstdecode the base layer, as noted above. Because the base layer istransmitted with higher reliability or more robustness than the enhancedlayer, it is likely that most mobile stations within the broadcast zonewill be able to receive and decode the base layer, whereas some mobilestations may not be able to both receive and decode the enhanced layer.The base layer is reconstructed by the receiver using the channelestimate information, as per block 1525. Receiver 490 then cancels thereconstructed base layer from the overall received signal, thusisolating the enhanced layer, as per block 1530. Receiver 490subsequently decodes the enhanced base layer, as per block 1535. Theoverall received signal may also be referred to as a composite signalbecause it may include both the base layer and the enhanced layer.

Mobile station receiver 490 performs a cyclic redundancy code (CRC)check on the decoded enhanced layer, as per decision block 1540. If theCRC check indicates that the integrity of the enhanced layer has beenpreserved, i.e. the CRC is OK, then receiver 490 may use both the baselayer and the enhanced layer to receive the video/audio or otherinformation signal transmitted to receiver 490, as per block 1545.Process flow stops at end block 1550 when reception of the broadcastinformation on the base layer and the enhanced layer is complete.However, if the CRC check fails, thus indicating that the integrity ofthe enhanced signal has not been preserved, then receiver 490 uses onlythe previously decoded base layer to receive the broadcasted informationfrom the base station. Process flow stops at end block 1550 whenreception of the base layer is complete. In this manner, a mobilestation receiver 490 can improve the quality of broadcast reception ofvideo images, audio and/or other information signals, when the signalstrength that the mobile station receives permits reception and decodingof both the base layer and the enhanced layer.

FIG. 16 shows another embodiment of the wireless communication system,namely system 1600, wherein some of the base stations employ 2 antennasand other base stations employ more than 2 antennas, for example 4antennas, for improved reception. Wireless communication system 1600includes elements in common with wireless communication system 400 ofFIG. 4A. Like numbers are used to indicate like elements when comparingthese two systems. In system 1600, broadcast/multicast content server410 provides information to broadcast/multicast controller 415.Controller 415 couples to base stations 1601, 1602 and 1603 to providethe information, now designated as information 420, to these basestations in their respective cells.

System 1600 includes base stations 1601, 1602 and 1603 that are situatedin the same broadcast zone, each base station being situated in arespective cell within the broadcast zone. Base stations 1601 and 1603each employ two antennas to transmit information. In contrast, basestation 1602 employs four antennas to transmit information in thisembodiment. More particularly, base station 1601 includes antennas 1611and 1612 for transmitting modulated symbols S1 and S2, respectively,whereas base station 1603 includes antennas 1621 and 1622 fortransmitting modulated symbols S1 and S2, respectively. Base station1602 employs four antennas to transmit information, namely antennas 1631and 1631′ to transmit modulated symbols S1, and antennas 1632 and 1632′to transmit the modulated S2 symbols.

The configuration of base stations 1601 and 1603 is similar to that ofbase stations 501 and 503 of FIG. 5. Base station 1601 includes ademultiplexer (DMUX) 1605 that provides two information streams tocoding and modulation stages 1610 and 1615, respectively. Coding andmodulation stages 1610 and 1615 respectively provide modulated signalsS1 and S2 to two antennas 1611 and 1612, respectively. Base station 1603includes a DMUX 1617 that provides 2 information streams to coding andmodulation stages 1620 and 1625, respectively. Coding and modulationstages 1620 and 1625 respectively provide modulated signals S1 and S2 toantennas 1621 and 1622, respectively.

Like the other embodiments discussed above, base stations 1601 and 1602transmit information via two spatially multiplexed streams on twoantennas, namely antennas 1611, 1612 in the case of base station 1601and antennas 1621, 1622 in the case of base station 1603. However, basestation 1602 employs 4 antennas to transmit two spatially multiplexedstreams from four antennas, namely antenna pair 1631, 1631′ for the S1modulated symbols and antenna pair 1632, 1632′ for the S2 modulatedsymbols. Base station 1602 employs cyclic diversity for each pair ofantennas to achieve such spatially multiplexed transmission from 4antennas as described below.

Base station 1602 employs a DMUX 1635 to split the information contentit receives from broadcast/multicast controller 415 into two informationstreams at its respective two outputs. Coding and modulation stage 1640codes and modulates one of these streams into symbols S1, while codingand modulation stage 1645 codes and modulates the other of these twostreams into symbols S2. The output of coding and modulation stage 1640couples to antenna 1631 as shown. The output of coding and modulationstage 1640 also couples to antenna 1631′ via a cyclic delay stage 1650therebetween. In this manner, the information stream containing the S1symbols is provided to antenna 1631 without delay, while the sameinformation stream is provided to antenna 1631′ with cyclic delay.Similarly, coding and modulation stage 1645 provides an informationstream containing the S2 symbols to antenna 1632, while also providingthe same information stream to antenna 1632 with the cyclic delay ofcyclic delay stage 1655. In another embodiment, random phase shifts maybe applied to the subcarriers transmitted from antennas 1631′ and 1632′.In such an embodiment, frequency-diversity may be obtained withoutintroducing cyclic delays.

Base station 1602 achieves the transmission of two spatially multiplexedinformation streams from four antennas by using cyclic delay diversityfrom each pair of two transmit antennas, namely antenna pair 1631, 1631′and antenna pair 1632, 1632′. Antenna pair 1631, 1631′ transmits theinformation stream containing the S1 symbols, while antenna pair 1632,1632′ transmits the information stream containing the S2 symbols. Theinformation stream containing the S1 symbols is transmitted over antenna1631′ with a cyclic delay provided by delay stage 1650, while theinformation stream containing S2 symbols is transmitted over antenna1632′ with a cyclic delay provided by delay stage 1655. The cyclic delayin these two streams provides additional frequency-diversity when thesystem is configured for single frequency network (SFN) operation usingorthogonal frequency division multiplexing (OFDM). It is noted that themobile station receiver 490 does not need to know if the transmissionfrom the base station is provided using 2 transmit antennas or 4transmit antennas because the cyclic delay of the delayed streamsappears as multi-path propagation to the mobile receiver 490.

FIG. 17 shows another embodiment of the wireless communication system assystem 1700. System 1700 is similar to system 1600 of FIG. 16, exceptfor the base station 1702 that system 1700 employs instead of basestation 1602. Base station 1702 transmits information streams over 4antennas, 1631, 1631′ and 1632, 1632′. Base station 1702 employsspace-time (ST) coding or space-frequency (SF) coding to achievetransmit diversity from the 4 transmit antennas. In more detail, basestation 1702 includes a space-time (ST) or space-frequency (SF) codingstage 1750 between coding and modulation stage 1640 and antenna pair1631, 1631′. ST/SF coding stage provides the S1 symbols to antennas1631, 1631′ after space-time or space-frequency coding those symbols.Base station 1702 also includes a space-time (ST) or space-frequency(SF) coding stage 1755 between coding and modulation stage 1645 andantenna pair 1632, 1632′. ST/SF coding stage 1755 provides the S2symbols to antenna pair 1632, 1632′ after space-time or space-frequencycoding those symbols.

One example of transmit diversity that base station 1702 of FIG. 17 mayemploy is an Alamouti 2×1 space-time (ST) diversity scheme such as shownin FIG. 18. More particularly, FIG. 18 shows the ST coding portion ofbase station 1702 as including a space-time (ST) coding stage 1750coupled to antennas 1631 and 1631′. Components to the left of dashedline 1800 represent base station 1702, namely the transmitter, andcomponents to the right of dashed line 1800 represent a portion of amobile station receiver 1805. In this approach, during any symbolperiod, two data symbols are transmitted simultaneously from the twotransmit antennas 1631 and 1631′, respectively. For example, during afirst symbol period or interval, antennas 1631 and 1631′ transmit thes(1) and s(2) symbols, respectively, as shown in FIG. 18. It is notedthat in FIG. 17, S1 and S2 denote two data streams. Returning to FIG.18, s(1) and s(2) are two consecutive symbols from the same data stream.Data stream s1 may further include symbols s1(1) and s1(2) as twoconsecutive symbols. During the next symbol period after the firstsymbol period, antennas 1631 and 1631′ transmit the symbols −s*(2) ands*(1), respectively, wherein s* represents the complex conjugate of s.With some processing at the receiver, as denoted by processing stages1810 and 1815, the receiver can recover the original symbols s(1) ands(2). Receiver 1805 includes an antenna 1820 that couples to one inputof an adder or summer 1825. Either an n(1) signal or an n(2) signal isprovided to the remaining input of adder 1825 as shown, wherein n(1) andn(2) are additive white Gaussian noise (AWGN) samples. Adder 1825 addsthe n(1) and n(2) noise samples to the incoming signals from antenna1820. Each of the signal paths between transmitter antenna 1631 andreceiver antenna 1820, and between transmitter antenna 1631′ andreceiver antenna 1820, exhibit channel gain which may vary over time.Receiver 1805 may employ a digital signal processor (DSP, not shown) toprepare instantaneous channel gain estimates g1 and g2 for the signalpaths from antenna 1631 and antenna 1631′. The system provides separatepilot symbols on both the antennas for channel gain estimation at thereceiver. The diversity gain achieved by Alamouti coding is the same asthat achieved in Maximum Ratio Combining (MRC). Moreover, a 2×1 Alamoutischeme may also be implemented in a space-frequency coded form. In thiscase, the two symbols are transmitted on two different frequencies, forexample, on different subcarriers in an Orthogonal Frequency DivisionMultiplexing (OFDM) system. In actual practice, adder 1825 andprocessing stages 1810 and 1815 may be combined in a common digitalsignal processor (DSP) integrated circuit or ASIC. For purposes of FIG.18, r1 and r2 are defined as follows: r1=g1s1+g2s2+n1 andr2=−g1s2*−g2s1*+n2. “v” is defined by the equations in processing stage1810 as shown in FIG. 18. Processing stage 1815 computes symbol decisionvariables s(1) and s(2).

FIG. 19 shows another embodiment of the wireless communication system assystem 1900. System 1900 includes base stations 1901, 1902 and 1903situated in respective cells that together form a broadcast zone. Basestation 1901 includes a coding and modulation stage 1905 that channelcodes and modulates the information it receives from broadcast/multicastcontroller 415. Coding and modulation stage 1905 provides a codedmodulated information stream at its output. The coding and modulationmethods described above may be employed in coding and modulation stage1905 to produce the coded modulation information stream. A diagonalBLAST coding stage 1910 couples to the output of coding and modulationstage 1905. Coding stage 1910 splits the coded modulation informationstream from the coding and modulation stage 1905 into two streams S1 andS2 that are diagonally BLAST (or D-BLAST) coded, wherein D-BLAST denotesDiagonal Bell Laboratories Layered Space-Time Architecture coding. Indiagonal BLAST, or D-BLAST, multiple codewords are staggered so thateach codeword spans multiple transmit antennas, but the symbols sentsimultaneously by the different transmit antennas belong to differentcodewords. Therefore, symbol coding in D-BLAST is performed across theantennas to obtain better performance. Antennas 1911 and 1912 transmitthe S1 and S2 symbol streams, respectively. Base stations 1902 and 1903exhibit substantially the same topology as base station 1901. In basestations 1902 and 1903, coding and modulation stages 1925 and 1945correspond to coding and modulation stage 1905 of base station 1901.Diagonal BLAST coding stages 1930 and 1950 correspond to diagonal BLASTstage 1910 of base station 1901. Base stations 1901, 1902 and 1903transmit the S1 information stream via corresponding antennas 1911, 1931and 1951, respectively. In a similar manner, base stations 1901, 1902and 1903 transmit the S2 information stream via corresponding antennas1912, 1932 and 1952, respectively. In one embodiment, the receiver 490receives first codeword (S1) and then cancels first codeword (S1) fromthe received signal. The receiver then decodes the second codeword (S2).

A wireless communication system is thus disclosed wherein an informationsource communicates with a mobile station via multiple intermediary basestations located in respective cells of a broadcast zone. In oneembodiment, each base station sends multiple information streams thatmay be received by a mobile station located in the broadcast zone. Byreceiving multiple information streams from multiple base stations, themobile station may enhance reception. In one embodiment, a base stationtransmits multiple information streams with different levels ofrobustness. The robustness of a particular transmitted informationstream may be determined for example by the particular coding of thetransmitted information stream, the particular modulation type of thetransmitted information stream and the particular power level of thetransmitted information stream.

In another embodiment, an information source communicates a base layerinformation stream and an enhanced layer information stream to multiplebase stations in a broadcast zone. The base layer information stream andthe enhanced layer information stream may each include the sameinformation content. However, the enhanced layer information stream mayinclude a higher resolution version of the information content than thebase layer information stream. Each base station transmits the baselayer information stream and the enhanced layer information stream. If amobile station in the broadcast zone receives base layer informationstreams and also reliably receives enhanced layer information streams,the mobile station may use both the enhanced layer information streamsand the base layer information streams to reconstruct the originalinformation content from the information source. However, if the mobilestation does not reliably receive enhanced layer information streams,then the mobile station reconstructs the original information contentfrom the received base layer information streams.

Modifications and alternative embodiments of this invention will beapparent to those skilled in the art in view of this description of theinvention. Accordingly, this description teaches those skilled in theart the manner of carrying out the invention and is to be construed asillustrative only. The forms of the invention shown and describedconstitute the present embodiments. Persons skilled in the art may makevarious changes in the shape, size and arrangement of parts. Forexample, persons skilled in the art may substitute equivalent elementsfor the elements illustrated and described here. Moreover, personsskilled in the art after having the benefit of this description of theinvention may use certain features of the invention independently of theuse of other features, without departing from the scope of theinvention.

1. A wireless communication system, comprising: an information sourcethat provides an information content stream having a base layer and anenhanced layer; and a plurality of base stations, each base stationcoupled to the information source and having a plurality of antennas,wherein each base station transmits from a first antenna a firstinformation stream derived from the base layer of the informationcontent stream and exhibiting a first level of robustness, each firsttransmitted information stream substantially identical to every otherfirst transmitted information stream, wherein each base stationtransmits from a second antenna a second information stream derived fromthe enhanced layer of the information content stream and exhibiting asecond level of robustness, each second transmitted information streamsubstantially identical to every other second transmitted informationstream, wherein at least one, but not all, of the base stationstransmits a portion of the first and second information streams fromthird and fourth antennas respectively using a cyclic delay, and whereinthe first level of robustness is achieved at least in part by a firstcoding rate and the second level of robustness is achieved at least inpart by a second coding rate, each coding rate being a ratio of inputbits to output bits at an encoder at each base station.
 2. The wirelesscommunication system of claim 1, wherein the first level of robustnessis greater than the second level of robustness.
 3. The wirelesscommunication system of claim 1, wherein each base station comprises aunitary pre-coding stage configured to perform unitary pre-coding on thefirst and second information streams.
 4. The wireless communicationsystem of claim 1, wherein each base station modulates the firsttransmitted information stream thereof with a first modulation type andeach base station modulates the second transmitted information streamthereof with a second modulation type, the first modulation typeexhibiting a higher level of robustness than the second modulation type.5. The wireless communication system of claim 1, wherein each basestation transmits the first transmitted information stream thereof at ahigher radio frequency output power than the second transmittedinformation stream thereof.
 6. The wireless communication system ofclaim 1, wherein a composite signal that includes first transmittedinformation streams and second transmitted information streams istransmitted to a mobile receiving station.
 7. The wireless communicationsystem of claim 1, wherein the base stations are situated in respectivecells.
 8. A method of receiving information by a mobile receivingstation, comprising: receiving a plurality of substantially identicalfirst information streams transmitted by a plurality of base stations,each of the first information streams derived from a base layer of aninformation content stream, each of the first information streamstransmitted by a first antenna of a different base station andexhibiting a first level of robustness; receiving a plurality ofsubstantially identical second information streams transmitted by theplurality of base stations, each of the second information streamsderived from an enhanced layer of the information content stream, eachof the second information streams transmitted by a second antenna of adifferent base station and exhibiting a second level of robustness;receiving cyclic delayed portions of the substantially identical firstand second information streams transmitted from third and fourthantennas respectively of at least one, but not all, of the basestations; testing the second information streams to determine if themobile receiving station reliably received the second informationstreams; and reconstructing the information content stream from thefirst information streams and the second information streams if thetesting step determined that the mobile receiving station reliablyreceived the second information streams, wherein the first level ofrobustness is achieved at least in part by a first coding rate and thesecond level of robustness is achieved at least in part by a secondcoding rate, each coding rate being a ratio of input bits to output bitsat an encoder at each base station.
 9. The method of claim 8, whereinthe first level of robustness is greater than the second level ofrobustness.
 10. A method of transmitting information, comprising:providing, by an information source, an information content stream to aplurality of base stations situated in respective cells, each basestation having a plurality of antennas, the information content streamhaving a base layer and an enhanced layer; transmitting from a firstantenna of each base station a first information stream derived from thebase layer of the information content stream and exhibiting a firstlevel of robustness, each first information stream substantiallyidentical to every other first information stream; transmitting from asecond antenna of each base station a second information stream derivedfrom the enhanced layer of the information content stream and exhibitinga second level of robustness, each second information streamsubstantially identical to every other second information stream; andusing a cyclic delay, transmitting a portion of the first and secondinformation streams from third and fourth antennas respectively of atleast one, but not all, of the base stations, wherein the first level ofrobustness is achieved at least in part by a first coding rate and thesecond level of robustness is achieved at least in part by a secondcoding rate, each coding rate being a ratio of input bits to output bitsat an encoder at each base station.
 11. The method of claim 10, whereinthe first level of robustness is greater than the second level ofrobustness.
 12. The method of claim 10, further comprising the step ofperforming, at each base station, unitary pre-coding on the first andsecond information streams.
 13. The method of claim 10, wherein eachbase station modulates the first transmitted information stream thereofwith a first modulation type and each base station modulates the secondtransmitted information stream thereof with a second modulation type,the first modulation type exhibiting a higher level of robustness thanthe second modulation type.
 14. The method of claim 10, wherein eachbase station transmits the first transmitted information stream thereofat a higher radio frequency output power than the second transmittedinformation stream thereof.
 15. A mobile receiving station configuredto: receive a plurality of substantially identical first informationstreams transmitted by a plurality of base stations, each of the firstinformation streams derived from a base layer of an information contentstream, each of the first information streams transmitted by a firstantenna of a different base station and exhibiting a first level ofrobustness; receive a plurality of substantially identical secondinformation streams transmitted by the plurality of base stations, eachof the second information streams derived from an enhanced layer of theinformation content stream, each of the second information streamstransmitted by a second antenna of a different base station andexhibiting a second level of robustness; receive cyclic delayed portionsof the substantially identical first and second information streamstransmitted from third and fourth antennas respectively of at least one,but not all, of the base stations; test the second information streamsto determine if the mobile receiving station reliably received thesecond information streams; and reconstruct the information contentstream from the first information streams and the second informationstreams if the testing step determined that the mobile receiving stationreliably received the second information streams, wherein the firstlevel of robustness is achieved at least in part by a first coding rateand the second level of robustness is achieved at least in part by asecond coding rate, each coding rate being a ratio of input bits tooutput bits at an encoder at each base station.
 16. The mobile receivingstation of claim 15, wherein the first level of robustness is greaterthan the second level of robustness.
 17. The mobile receiving station ofclaim 15, wherein the mobile receiving station is further configured toperform a cyclic redundancy check (CRC) on at least one of the secondinformation streams to determine if the at least one second informationstream was reliably received.
 18. The mobile receiving station of claim15, wherein each first information stream is coded with a first code andeach second information stream is coded with a second code, the firstcode exhibiting a higher level of robustness than the second code. 19.The mobile receiving station of claim 15, wherein each first informationstream is modulated with a first modulation type and each secondinformation stream is modulated with a second modulation type, the firstmodulation type exhibiting a higher level of robustness than the secondmodulation type.
 20. The mobile receiving station of claim 15, whereineach first information stream is transmitted at a higher radio frequencypower level than each respective second information stream.
 21. Themobile receiving station of claim 18, wherein the first informationstreams and the second information streams together form a compositesignal, the mobile receiving station further configured to: receive thecomposite signal; decode the first information streams to provide adecoded first information stream; cancel the decoded first informationstream from the composite signal to provide a received secondinformation stream; and decode the received second information stream.